Block self-cross-linkable poly(ethylene terephthalate) copolyester via solid-state polymerization: Crystallization, cross-linking, and flame retardance

Article abstract of DOI:10.1016/j.polymer.2015.06.012

Abstract A self-cross-linkable poly(ethylene terephthalate) block copolyester (SSP-PETPx) has been synthesized via solid-state polymerization (SSP). In the SSP process, a cross-linkable monomer named 4-(phenylethynyl) di(ethylene glycol) phthalate (PEPE) is incorporated into the amorphous phase of PET. The sequence distribution of the resulted copolyester SSP-PETPx is analyzed with ¹H NMR, showing that the SSP sample possesses block structure. Wide-Angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) show that SSP-PETPx is crystallizable benefitting from its block structure, while the corresponding random copolyester (RD-PETP) is totally amorphous. The segments containing PEPE determine the extent of crystallization. Simultaneous thermogravimetric-differential scanning calorimetry (TG-DSC) and rheological analysis prove the cross-linking behaviour of SSP-PETP although the block constitution slightly postpones cross-linking occurrence. The high complex viscosity of the SSP-PETP at 340 °C suggests good flame retardancy and anti-dripping properties. Micro combustion calorimeter (MCC) results prove both SSP-PETP and RD-PETP copolyesters have low flammability, which demonstrates that block structure of SSP-PETP doesn't have a negative impact on the flammability of copolyesters.

Full text of DOI:10.1016/j.polymer.2015.06.012

Polymer 70 (2015) 68e76
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Block self-cross-linkable poly(ethylene terephthalate) copolyester via
solid-state polymerization: Crystallization, cross-linking, and flame
retardance
,
,
Hai-Bo Zhao a b, Xiao-Lin Wang a, Yue Guan a, Xiu-Li Wang a *, Li Chen a,
Yu-Zhong Wang a
,
*
a Center for Degradation and Flame-Retardant Polymeric Materials, College of Chemistry, State Key Laboratory of Polymer Materials Engineering, National
Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, Chengdu 610064, PR China
b Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A self-cross-linkable poly(ethylene terephthalate) block copolyester (SSP-PETPx) has been synthesized
via solid-state polymerization (SSP). In the SSP process, a cross-linkable monomer named 4-(phenyl-
ethynyl) di(ethylene glycol) phthalate (PEPE) is incorporated into the amorphous phase of PET. The
sequence distribution of the resulted copolyester SSP-PETPx is analyzed with 1H NMR, showing that the
SSP sample possesses block structure. Wide-Angle X-ray diffraction (WAXD) and differential scanning
calorimetry (DSC) show that SSP-PETPx is crystallizable benefitting from its block structure, while the
corresponding random copolyester (RD-PETP) is totally amorphous. The segments containing PEPE
determine the extent of crystallization. Simultaneous thermogravimetric-differential scanning calorim-
etry (TG-DSC) and rheological analysis prove the cross-linking behaviour of SSP-PETP although the block
constitution slightly postpones cross-linking occurrence. The high complex viscosity of the SSP-PETP at
340 ꢀC suggests good flame retardancy and anti-dripping properties. Micro combustion calorimeter
(MCC) results prove both SSP-PETP and RD-PETP copolyesters have low flammability, which demon-
strates that block structure of SSP-PETP doesn't have a negative impact on the flammability of
copolyesters.
Received 17 December 2014
Received in revised form
21 May 2015
Accepted 10 June 2015
Available online 12 June 2015
Keywords:
Block copolyester
Solid-state polymerization
Crystallization
Cross-linking
Flame retardance
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
leading to secondary damage and immediate empyrosis [13e17]. It
is difficult to solve the contradiction between the flammability and
Poly(ethylene terephthalate) (PET), a partly aromatic semi-
crystalline polyester, has wide applications in our daily life such as
beverage bottle, fiber and packing materials [1e3]. However, its fire
risk seriously threatens human lives and property due to its flam-
mable nature [4e6]. To obtain flame-retardant polyester material,
conventional methods are incorporating flame-retardant elements
into their main chains or side chains by polymerization, or adding
flame retardants to their matrices [7e9]. Among them, copoly-
merizing phosphorus-containing monomers into polyester chains
is considered to be one of the most efficient methods for prepara-
tion of flame-retardant fiber [10e12]. However, most phosphorus-
containing copolyesters cause serious melt-dripping during fire,
dripping behaviour for polyester via copolymerization.
To overcome this problem, in our recent work, a kind of cross-
linkable PET-based copolyester was designed and synthesized
[18]. This copolyester is inactive at the temperature of synthesis and
processing. But it can cross-link rapidly at higher temperature
before burning, which endows copolyester with self-
extinguishment and non-dripping behaviour. Besides, this ther-
mal cross-linkable copolyester possesses only carbon, hydrogen,
and oxygen elements, which is a truly green and environment-
friendly material. However, in order to obtain high flame-
retardancy and pass the UL-94 or LOI tests, high content of func-
tional monomer is needed for this copolyester [19]. Without a
doubt, the regularity and symmetry of polymer chain sharply de-
creases especially for the copolyesters with high content of func-
tional monomer, and this will low their crystalline ability. For
example, when the content of functional monomer is 16.3%, its
* Corresponding authors.
E-mail addresses: xiuliwang1@163.com (X.-L. Wang), yzwang@scu.edu.cn
(Y.-Z. Wang).
http://dx.doi.org/10.1016/j.polymer.2015.06.012
0032-3861/© 2015 Elsevier Ltd. All rights reserved.

H.-B. Zhao et al. / Polymer 70 (2015) 68e76
69
melting enthalpy is decreased from 43.1 (for pure PET) to 6.1 J gꢁ1
[18]. And it is well known that the crystalline ability of polymer has
great relationship with its mechanical properties. Therefore, it is a
key issue for this copolyester keeping good crystallinity and self
cross-linking properties simultaneously.
Solid-state polymerization (SSP) is an effective method to syn-
thesize copolyester with block constitution [20e22]. For polyester,
SSP, is performed at the temperature below its melting temperature
and higher than its glass transition temperature, which is usually
used to increase the molecular weight. SSP occurs in the amor-
phous region of polyester while the chain motion in crystalline
regions is restricted [23e27]. Based on this feature, the functional
monomer can be incorporated into the amorphous regions of
polyester via SSP, and the resulted copolyester possesses block
powder (particle size: 1.0e1.8
m
m, Fig. S1). Finally, the powdered p-
PET/PEPE mixture was dried in oven at 80 ꢀC for 24 h prior to
further use.
2.3. Solid-state polymerization (SSP)
SSP was carried out in a reaction tube under reduced pressure.
The temperature was kept at 190 ꢀC for 2 h, and then was raised to
205 ꢀC. The tube was about 120 mm long, with the diameter of
15 mm. Reaction pressure was maintained at about 20e25 Pa, and
reaction time were 0e8 h. The resulting SSP copolyesters are
abbreviated as SSP-PETPx (Scheme 1), where the number x denotes
the molar parts of PEPE per hundred of DMT. In this work, x is 30,
40, 60 or 80.
constitution. Via SSP, Chen et al. [20,28] prepared
a block
phosphorus-containing poly(trimethylene terephalate) copolyester
using the blends of 9,10-dihydro-10-[2,3-di(hydroxycarbonyl)pro-
pyl]-10-phospha-phenanthrene-10-oxide (DDP) ester and poly(-
trimethylene terephalate) (PTT) oligomer as raw materials. The
crystallization behaviours of copolyesters were improved.
In this paper, self-cross-linkable block copolyesters were also
synthesized via SSP. In SSP process, a cross-linkable monomer
named 4-(phenylethynyl) di(ethylene glycol) phthalate (PEPE) was
incorporated into the amorphous phase of PET. The sequence dis-
tribution and the randomness of the obtained copolyesters were
analyzed by NMR. The detailed crystallization properties of copo-
lyesters were investigated using DSC and WAXD. The cross-linking
behaviours of the resulting copolyesters were discussed by TG-DSC
and dynamic rheology. The relationship between the thermal be-
haviours and block constitution was investigated in detail. The
flammability of the obtained copolyesters was examined by MCC.
2.4. Melt polycondensation
For comparison, the random self-cross-linkable copolyester
(RD-PETP40) and neat PET were prepared via one-pot melt poly-
condensation according to our previous work [18].
2.5. Characterization
NMR spectra of copolyesters (1H, 600 MHz; 13C, 400 MHz) with
CF3COOD as the solvent, and tetramethylsilane as the internal
stand, were obtained at room temperature by Bruker AVANCE
AVⅡ600 NMR and Bruker AVANCE AVⅡ400 instrument, respectively.
The intrinsic viscosities of p-PET were measured with an Ubbe-
lohde viscometer with a concentration of 0.5 g/dL at 25 ꢀC in 1:1 (v/
v) phenol-1,1,2,2-tetrachloroethane solution. The content of PEPE
incorporated in PET chain was determined by 1HNMR. Before NMR
tests, all samples dissolved in CF3COOH with stirring for 30 min to
make a solution, and then were precipitated by methanol. Via this
treatment the unreacted PEPE was removed. The obtained products
were dried in oven until constant weight.
2. Experimental
2.1. Chemicals and substrates
Dimethyl terephthalate (DMT, CP grade) was provided by
Sinopharm Chemical Reagent Co. Ltd (Shanghai China). Ethylene
glycol (EG), antimony trioxide (Sb2O3, AR), methanol, phenol, tet-
rachloroethane, hexafluoro-isopropanol (HFIP), zinc acetate (CP)
were obtained from Chengdu Chemical Industries Co. (Chengdu,
China) and used as received. 4-phenylethynylphathalic anhydride
(PEPA) was purchased from Changzhou Sunlight Pharmaceutical
Co., Ltd. (Changzhou, China).
2.6. Crystallization characterization
The thermal transition behaviours of RD-PETPx and SSP-PETPx
were measured with TA Q200 DSC apparatus, calibrated with
pure indium and zinc standards. Samples (5 0.5 mg) were first
heated to 270 ꢀC for 3 min, to eliminate the influence of thermal
history and the effect of heat treatment on the crystalline structure
of materials. Then, the sample was cooled down to 40 ꢀC to record
the crystallization process, and reheated to 270 ꢀC at a heating rate
of 10 ꢀC/min. WAXD measurements of RD-PETP40 and SSP-PETP40
were performed using an X-ray diffractometer (Philips X Pert X-ray
2.2. Pre-polymerization
PET prepolymer (p-PET) was prepared by transesterification
method (Scheme 1). DMT and EG with molar ratio of 1:4 were
added into a reaction apparatus with a nitrogen inlet, a conden-
sation, and a mechanical stirrer. Transesterification catalyst zinc
acetate and polycondensation catalyst Sb2O3 were also added
before reaction. The mixtures were kept at 180 ꢀC with nitrogen
protection and mechanical stir. After about 2 h, methanol was
distilled out completely. Then, the reaction was further carried out
under reduced pressure (40 Pa) for half an hour at 270 ꢀC. 1H NMR
diffractometer), with Cu Ka radiation in a 2q
ranges from 2 to 50ꢀ.
All samples were annealed at 160 ꢀC for 2 h before testing.
2.7. Cross-linking behaviour
Cross-linking behaviours of copolyesters were investigated by a
NETZSCH simultaneous TGA-DSC (449C) at a heating rate of
10 ꢀC minꢁ1 in N2. Dynamic oscillatory rheological measurements
of RD and SSP copolyesters were performed with a parallel-plate
fixture (25 mm diameter and 1 mm thickness) using an Advanced
Dynamic Rheometric Expansion System (ARES, Bohlin Gemini 200)
in an oscillatory shear mode. Temperature scanning tests at a fixed
(CF3COOD, d): 8.4 (AreH), 5.0 (eC(O)OeCH2CH2eOe(O)Ce), 4.2
(-CH2-OH). The intrinsic viscosity of resulting p-PET was 0.19 dL/g.
Cross-linkable monomer PEPE was synthesized as our previous
work [18]. PEPA was firstly ethoxylated in the presence of ethylene
glycol (Scheme 1). 1H NMR (400 MHz, DMSO-d6,
d): 7.6e8.5 (AreH),
5.0e5.4 (eC(O)OeCH2-), 4.3e4.6 (-CH2-OH).
p-PET and PEPE were dissolved into HFIP at 45 ꢀC. After com-
plete dissolution, the temperature was raised to 90 ꢀC to remove
the HFIP. Then, the obtained p-PET/PEPE mixture was ground into
frequency of 1 rad/s in air were in the range of 200 or
240 ꢀCe340 ꢀC for RD and SSP samples at a heating rate of
10 ꢀC minꢁ1, respectively.

70
H.-B. Zhao et al. / Polymer 70 (2015) 68e76
Scheme 1. Solid-state polycondensation route of SSP-PETPx.
2.8. Flammability
polymer where the chains are mobile, as shown in Scheme 2. This
block SSP-PETPx consisted of pure homopolymer polyester block
(only TET repeat units) mainly in the crystalline domains and
copolyester (both TEP and PEP units) in the amorphous parts. The
chemical structure of SSP-PETP40 was determined by 13CNMR and
1HNMR. Fig. 1 showed the 13CNMR spectrum of SSP-PETP40
(tSSP ¼ 8 h), where the resonance signals occur at 87, 94, and
122e132 ppm were ascribed to the phenylethynyl groups. Fig. 2
showed the 1HNMR spectra of SSP-PETP40 (tSSP ¼ 8 h) and RD-
PETP40. Each spectrum was subjected to peak fitting using the curve
fitting software MestReNova 6.1.0e6224. The resonance signal
occurred at 8.25 ppm (a) was ascribed to the aryl of DMT (T), and the
signals at 7.25e7.90 (b) ppm were assigned to the aryl of PEPE (P).
During SSP, an ester bond will form when PEPE is incorporated into
polymer chains. But PEPE has the same ester bond itself, that is to
say, PEPE and SSP-PETP40 have very similar phenylethynyl group
signals in NMR spectra, so, we cannot get the direct information
about covalent links from their NMR spectra. Because the unreacted
PEPE had been removed before NMR test, the signals of phenyl-
ethynyl groups in NMR spectrum of SSP-PETP40 were ascribed to the
PEPE segment in the copolyesters. And this illustrated that PEPE had
been successfully incorporated into the copolyester chain via cova-
lent bond. Especially, the signals at 4.65e5.05 in the Fig. 2 were
caused by the methylene of EG with different combinations with
DMT or PEPE. The possible combinations of TET, TEP (PET), and PEP
were shown in Scheme 3. And the peaks at 4.92e5.05, 4.82e4.92,
and 4.65e4.82 ppm were assigned to the methylene of TET (c), TEP
(d), and PEP (e), respectively. In particular, the signals of PEP units
only existed in SSP-PETP40, indicated that PEPE accumulated into
homogeneous sequences in SSP. However, PEP signals cannot be
observed in RD-PETP40. Actually, the concentration of PEPE units in
Small scale flammability tests were performed on the FTT 0001
MCC Microscale Combustion Calorimeter (MCC). 5e10 mg samples
were heated to 750 ꢀC from 90 ꢀC at a ramp rate of 1 ꢀC/s in a stream
of nitrogen flowing at 80 mL/min. The combustor temperature ꢁw1as
set at 900 ꢀC and oxygen/nitrogen flow rate was 20/80 mL mL
3. Results and discussion
.
3.1. Solid-state polymerization
To study the solid-state polymerization process, SSP-PETP40
with solid polymerization time (tSSP) from 0 to 8 h was carried out.
As shown in Scheme 1, transesterification reaction between PEPE
and p-PET was main chemical reaction during SSP, and the chem-
ical structure of the obtained copolyesters consisted of E-T and E-P
unit, in which E stood for EG unit, T stood for DMT unit, and P stood
for PEPE unit. The content of PEPE incorporated in PET chain was
determined by 1HNMR. Table 1 presents the change of reaction
extent with tSSP during SSP. The cross-linkable monomer PEPE
showed a quick reaction with p-PET during the first 2 h reflected by
the rapid increase of PEPE actual content. When tSSP reached 8 h,
almost all free PEPE monomer had been incorporated in the
copolyester via transesterification reaction. At this time the actual
PEPE content is 27.0%, which is almost as same as that of RD-PETP40
.
3.2. Structure and sequential distribution analysis
As our design, the block constitution of SSP-PETPx is formed
during SSP, because SSP only occurs in the amorphous area of
Table 1
Basic characteristic of SSP-PETP40 and RD-PETP40 copolyesters.
Samples
tSSP (h)
PEPE content (mol %)
Intensity of chemical shifts (%)
PTP
PPT
LnT
LnP
R
Theo.
28.6
Actuala
c
d
e
SSP-PETP40
1
2
4
8
e
17.3
23.1
27.0
27.0
28.0
77.7
68.5
62.0
60.6
43.9
10.0
17.0
21.8
24.8
56.1
12.3
14.6
16.2
14.6
0
0.06
0.11
0.15
0.17
0.39
0.28
0.37
0.41
0.46
1
16.7
9.1
6.7
5.9
2.6
3.6
2.7
2.4
2.2
1
0.34
0.48
0.56
0.63
1.39
RD-PETP40
28.6
a
Actual PETP contents were calculated from H NMR.

H.-B. Zhao et al. / Polymer 70 (2015) 68e76
71
Scheme 2. Schematic representation of solid-state polymerization process of SSP-PETPx: (a) before SSP, (b) after SSP, (c) the formed SSP-PETPx block copolyester.
the amorphous phase during SSP is higher than its concentration in
Id
ITEP
1
LnP
the melt during melt copolymerization, thereby enhancing the
chance of PEP sequence formation. And this further demonstrates
that our design is successful and block constitution of SSP-PETP40 is
obtained.
Furthermore, the degree of randomness of PEPTx named R was
calculated by 1HNMR, and the compositions of each sequence were
determined by Equations (1)e(3) [29e31].
2
2
PPT
¼
¼
¼
(2)
ITEP
2
Id
þ IPEP
2 þ Ie
R ¼ PTP þ PPT
(3)
(4)
ITEP
Id
2
IT ¼ ITET
þ
¼ Ic þ
¼ Ie þ
2
ITEP
Id
ITEP
Id
2
1
LnT
2
2
IP ¼ IPEP
þ
(5)
PTP
¼
¼
¼
(1)
ITEP
2
Id
2
þ ITET
2 þ Ic
Where IT (Ia), IP (Ib), ITET (Ic), ITEP (Id), and IPEP (Ie) corresponded to
the integrated intensities of a, b, c, d, and e, respectively. The PTP
stood for the probability of finding a P unit next to a T unit, and the
PPT stood for the probability of finding a T unit next to a P unit. LnT
meant the number-average sequential length of TET unit, and LnP
meant the number-average sequential length of PEP unit. In order
to reduce the error, ITET, IT, and IP were directly obtained from
1HNMR peak integration, while ITEP and IPEP were calculated by
Equations (4) and (5).
As we know, the lower degree of randomness (R) denoted the
units of copolymer tend to cluster together into blocks. In our
investigation, the R value of SSP-PETP40 (tSSP ¼ 8 h) was 0.63, while
the value for RD-PETP40 with random structure was 1.39. It was
clear that the repeating units of SSP-PETP40 tend to accumulate into
homogeneous sequences such as T-E-T-E-T or P-E-P-E-P, or block
constitution, in another word [32,33].
The degree of randomness as a function of reaction time was
also calculated and listed in Table 1. From Table 1, it was found that
PTP and PPT of SSP-PETP40 increased with the increase of tSSP
,
resulted in an increase of the degree of randomness. During SSP, the
amount of PEPE incorporated in the polymer chains increased, and
at the same time TET combination decreased. And TEP combination
greatly increased, however, PEP combination increased a little.
Thus, the degree of randomness increased. That is to say, the
transesterification reaction between PEPE and p-PET in the
Fig. 1. 13C NMR spectrum of SSP-PETP40 (tSSP ¼ 8 h).

72
H.-B. Zhao et al. / Polymer 70 (2015) 68e76
Fig. 2. 1H NMR spectra of SSP-PETP40 (tSSP ¼ 8 h) (A), and RD-PETP40 (B).
Scheme 3. Sequence distribution of T and P unit.
amorphous region plays an important role in increasing the R value
[20,34]. The obtained copolyester kept the original crystalline re-
gion of PET, however, the new amorphous area actually became
copolyester. With the increase of tSSP, the copolyesters possessed
more enough time for transesterification, resulting in more random
copolyeser formation.
with same thermal conditions to maintain the same crystallization
environment. SSP-PETP40 showed a typical diffraction pattern as
same as that of PET (Fig. 3) [5,35e38], indicating the introduction of
PEPE segment didn't change the crystal structure of PET. For RD-
PETP40 obtained from melt polycondensation, a broad peak was
observed indicated it lost crystalline ability and became amor-
phous. This illustrates, as we expected, SSP-PETP copolyester with
the block constitution is crystallizable, which is beneficial to the
application in the future [39,40].
3.3. Crystallization behaviour
In order to investigate the crystallization behaviour of SSP-
PETPx, WAXD measurement was used for SSP-PETP40 (tSSP ¼ 8 h)
and RD-PETP40 (Fig. 3). Before testing, all samples were treated
The thermal transition behaviors of SSP-PETP40 with different
tSSP are shown in Fig. 4. The detailed data are listed in Table 2.
According with XRD results, SSP-PETP40 showed obvious

H.-B. Zhao et al. / Polymer 70 (2015) 68e76
73
Fig. 3. WAXD patterns of SSP-PETP40 (tSSP ¼ 8 h) and RD-PETP40
.
crystallization and melting peaks, while RD-PETP40 was completely
amorphous and only glass transition temperature was found. The
thermal transition behaviors of the blends of p-PET and PEPE were
also investigated for comparison. Only a weak melting and crys-
tallization peak ascribed to p-PET was found in DSC heating and
cooling scan of the blends. This phenomenon illustrated there are
no reaction occurrence when PEPE with p-PET was only simply
blended. When the SSP was in progress, the reaction only occurred
in the amorphous area of p-PET, which resulted in the constituents
of crystallizable chain (part of TET) unchanged (Scheme 2). Besides
this, more TEP combination formation due to the trans-
esterification, and made the steric hindrance on crystallization
caused by PEPE monomers alleviated. As a result, the obtained
copolyester exhibited a good crystalline property, which was re-
flected by the increase of their melting point (Tm), crystallization
temperature (Tc), and melting enthalpies with the increase of tSSP
.
Moreover, the PEPE content also affect the crystallization
properties of SSP-PETPx. The thermal transition behaviors of SSP-
PETPx (tSSP ¼ 8 h) with different PEPE content were investigated
by DSC (Fig. 5 and Table 3). It was found that the crystallization
ability was decreased with the increase of PEPE content, embodied
Fig. 4. DSC thermograms of RD-PETP40 and SSP-PETP40 with different tSSP and RD-
PETP40: (a) the second heating scan, (b) the cooling scans at rate of 10 ꢀC minꢁ1
.
by the lower Tm, Tc and
DHm. Especially for SSP-PETP80, it did not
thermogram of SSP-PETP40 (Fig. 6), a notable exothermal process
started at 336 ꢀC was found, which is just between the melting and
the decomposition peak [41e45]. This means at this temperature
the cross-linking reaction occurred. The cross-linking reaction of
copolyesters is caused by the cross-linking reaction of phenyl-
ethynyl groups, in which two phenylethynyl groups reacted
together and formed one triphenylnaphthalene cross-links [19].
Besides, a melting peak also can be obviously seen in its DSC curves.
On the contrary, only cross-linking and decompositon peak were
found for RD-PETP40. It was clear that after SSP, the copolyester kept
crystallization and cross-linking property simultaneously, just as
shown in Scheme 4.
show any crystallization peak in its cooling scan, and only very faint
melting peak presented in its heating curve. It is well known that
the relative crystallinity of polymer can be determined by the ratio
of melting enthalpy (
polymer. At present, we cannot obtain the
crystalline SSP-PETPx, and the detailed crystallinity of the obtained
products did not be determined. But from the lower value of Hm
D
Hm) of samples to that of 100% crystalline
D
Hm value of 100%
D
we can sure that the crystallinity was also decreased with the in-
crease of PEPE content. The poor crystallization ability was due to
the big steric hindrance of PEPE monomers, which retarded the
chain motion of copolyester even it only appeared in the amor-
phous region of the obtained products. Fortunately, the obtained
copolyester (such as SSP-PETP60) with relative high PEPE contents
still showed an obvious crystallization and melting peak.
Table 2
Thermal transition data of RD-PETP40 and SSP-PETP40 with different tSSP
.
Tg (ꢀС)
Tc (ꢀС)
D
Hc (J gꢁ1
)
Tm (ꢀС)
D )
Hm (J gꢁ1
3.4. Cross-linking behaviour
Sample
RD-PETP40
Blends (0h)
SSP-PETP40-2h
SSP-PETP40-4h
SSP-PETP40-8h
81.7
58.1
75.0
79.3
80.1
e
e
e
e
The cross-linking behaviours of SSP-PETPx were studied by
TG-DSC simultaneous thermal analysis and dynamic rheometer, in
which SSP-PETP40 (tSSP ¼ 8 h) and RD-PETP40 were chosen as a
127.7
149.7
159.8
170.4
2.1
195.2
207.8
229.0
233.5
6.5
9.2
12.3
22.0
13.0
16.2
20.9
representative and
a control, respectively. From the DSC

74
H.-B. Zhao et al. / Polymer 70 (2015) 68e76
Fig. 6. TG-DSC thermograms of SSP-PETP40 (tSSP ¼ 8 h) and RD-PETP40 at rate of
10 ꢀC minꢁ1 in N2.
Similar with random self-cross-linking copolyester, the h* of SSP-
PETP40 experienced a decrease firstly and subsequently increased
sharply at the cross-linking temperature, at which * starts to in-
h
Fig. 5. DSC thermograms of SSP-PETPx (tSSP ¼ 8 h): (a) the second heating scan, (b) the
crease or cross-linking reaction occurs in other words [46,47]. And a
‘U-shape’ change was clearly observed in the whole temperature
range. Besides this, SSP-PETP40 with block constitution had higher
cross-linking temperature than that of RD-PETP40, which was
accord with the results of TG-DSC. From Fig. 7, it can also be seen
that compared with RD-PETP40, SSP-PETP40 showed a relatively low
melt viscosity at the whole investigated temperature. This meant
cooling scans at rate of 10 ꢀC minꢁ1
.
If we observed carefully, some differences can be found in Fig. 6.
The cross-linking peak of RD-PETP40 started at 314 ꢀC, however, the
cross-linking peak of SSP-PETP40 occurred later at about 336 ꢀC.
This means the block constitution of copolyester after SSP inhibited
the cross-linking occurrence slightly. As we know, for SSP-PETPx,
the cross-linkable chains concentrated at amorphous region,
which made copolyester having big steric hindrance than that of
RD-PETP. The bigger steric hindrance meant that the cross-linking
groups were more difficult to contact, that is, cross-linking reac-
tion is hard to happen.
that the increase of h* of SSP-PETP40 became difficult due to its
block constitution. But the complex viscosity of SSP-PETP40 at the
temperature of 340 ꢀC was still very high (more than 10000 Pa s),
illuminated it will endow the copolyester with good flame retard-
ancy and anti-dripping. And this will be investigated later.
3.5. Flammability
Furthermore, as the most significant effect caused by self-
thermal cross-linking, the complex viscosity (h*) of the testing
The flammability characteristics of RD-PETP40 and SSP-PETP40
were examined by MCC, which is a new and useful method
developed in recent years for investigating the combustion prop-
erties of polymer materials [51,52]. And neat PET was also inves-
tigated for comparison. Several parameters, such as heat release
rate capacity (HRC), total heat release (THR) and temperature at the
maximum heat release rate (Tmax), were obtained and presented in
Table 4. HRC is the peak heat release rate (HRR) normalized to the
heating rate, and low values of HRC indicate low flammability and
low full-scale fire hazard. As shown in the HRR curves (Fig. 8), neat
PET underwent heat release with a sharp peak, and exhibited a high
samples during the heating process, which play a crucial role in
flame retardancy and anti-dripping [46e50], was investigated by
dynamic oscillatory rheological measurements in air (Fig. 7).
Table 3
Thermal transition data of SSP-PETPx (tSSP ¼ 8 h).
Sample
Tg (ꢀС)
Tc (ꢀС)
D
Hc (J gꢁ1
)
Tm (ꢀС)
D )
Hm (J gꢁ1
SSP-PETP30
SSP-PETP40
SSP-PETP60
SSP-PETP80
79.0
80.1
81.6
80.9
190.4
170.4
156.0
e
26.3
22.0
5.4
e
234.7
233.5
225.8
221.7
21.0
20.9
9.5
1.4
HRC value of 472 J gꢁ1 K. In the case of RD-PETP40 and SSP-PETP40
,

H.-B. Zhao et al. / Polymer 70 (2015) 68e76
75
Scheme 4. Schematic representation of cross-linking and crystallization process of SSP-PETPx.
Fig. 7. Complex melt viscosity of SSP-PETP40 (tSSP ¼ 8 h) and RD-PETP40 plotted against
Fig. 8. Heat release rate curves of neat PET, RD-PETP40, and SSP-PETP40 (tSSP ¼ 8 h).
temperature at rate of 10 ꢀC minꢁ1 in an air atmosphere.
[18]. As mentioned earlier, the cross-linking reaction of SSP-PETPx
is just slightly postponed, and SSP-PETP40 and RD-PETP40 show
very similar cross-linking behaviours. Consequently, SSP-PETP40
two heat release peaks with a broad region were observed.
Meanwhile, compared with neat PET, they showed much lower
HRC values of 303 J gꢁ1 K and 311 J gꢁ1 K, respectively, indicating
they have lower flammability. Also, RD-PETP40 and SSP-PETP40 had
lower THR values of 15.8 kJ gꢁ1 and 16.0 kJ gꢁ1, while neat PETgive a
value of 17.2 kJ gꢁ1. It meant that more volatile products were
carbonized to participate in the charring process, rather than
transferred into the MCC combustor [53].
In addition, it is noteworthy that the HRR curve of SSP-PETP40
almost coincides with that of RD-PETP40, and they have similar HRC
and THR values. This illustrates that no matter what structure of
PETP40 has, it almost exhibit same flame retardance. That is to say,
the block structure of SSP-PETP40 doesn't have a negative impact on
the flammability of copolyesters. Actually, the action of cross-
linking monomer in reducing the flammability of polymers is
attributed to the improving the melt viscosity and compact char
has low flammability just as RD-PETP40
.
4. Conclusion
In summary, self-cross-linkable block copolyesters have been
successfully synthesized using PET prepolymer and PEPE as raw
materials via SSP. 1H NMR analysis shows the SSP-PETPx copo-
lyester has a block chemical structure, i.e. one block is “PET crys-
talline region”, and another one is “P(ET-co-P) random copolyester
region”. Transesterification reaction only occurs in the amorphous
phase of p-PET. With the decrease of tSSP, the degree of randomness
of copolyesters decreases. WAXD and DSC results demonstrate that
after SSP the copolyesters are crystallizable, even for the samples
with high PEPE content. This can be ascribed to their block struc-
ture, while the corresponding random copolyester is totally amor-
phous. The segments containing PEPE in amorphous area of SSP-
PETPx determines its crystallization ability. The higher PEPE con-
tent introduced into PETPx copolyester lowers its crystallization
ability. The SSP-PETPx keep the cross-linking properties although
the cross-linking reaction has been slightly postponed compared
with random copolyester. The complex viscosity of SSP-PETPx at
Table 4
MCC data of neat PET, RD-PETP40, and SSP-PETP40 (tSSP ¼ 8 h).
Sample
HRC (J gꢁ1 K)
THR (J gꢁ1 K)
Tmax (ꢀС)
PET
RD-PETP40
SSP-PETP40
472
303
311
17.2
15.8
16.0
442.8
437.1
439.6

76
H.-B. Zhao et al. / Polymer 70 (2015) 68e76
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Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (50933005, 51421061), and the
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